The present invention relates to an assembly for mixing or blending cold water with overheated hot water to form a stream of hot water at a desired temperature. In particular the present invention relates to a control valve assembly which is capable of maintaining a stream of blended hot water at a nearly constant temperature over a wide range of flow rates of blended hot water.
In hot water heating systems, particularly in institutional systems, it may be practical to heat water with a supply of steam. In such a system cold water is passed through one set of passages in a heat exchanger and is heated by the steam which passes through an adjoining set of passages. The output from the heat exchanger is overheated, that is, it is much too hot to be safe for use at a hot water tap. This overheated water is blended with cold water in the water heater until the resultant mixture is at a suitable temperature for the hot water tap.
There have been previous attempts to provide a controller which is capable of maintaining the temperature of the blended water constant over a wide range of flow rates. Some of these have used sleeve valves. Problems have been encountered when using sleeve valves to regulate water flow because the machined surfaces of those valves tend to corrode and to accumulate deposits making them difficult to operate. In addition, the constant contact between the sliding and fixed members of a sleeve valve creates friction which must be overcome to change the valve position. Some patents disclosing the use of sleeve valves in blending hot water heaters are U.S. Pat. Nos. 3,670,807, 3,388,861, and 3,232,336.
Further problems have been encountered when a flow of heated water is regulated by passing the hot water through a variable orifice. This can be explained by the principle that heated water passing through an orifice of the proper size will regulate its own flow. The size of the orifice establishes a reference temperature. If the temperature of the heated water exceeds the reference temperature, the flow through the orifice is choked off by cavitation or flashing downstream of the orifice. Choking continues until the temperature drops down to the reference temperature.
This choking phenomenon can be dangerous, since the result of choking off the flow is a rise in the pressure upstream of the valve or orifice. If the heat exchanger is upstream of the flow regulating valve, the pressure rise may cause the heat exchanger to explode. Blending hot water heaters having valves to regulate the flow of hot water downstream of the heat exchanger are disclosed in each of the above mentioned patents and in U.S. Pat. No. 2,610,837.
Difficulties have been encountered in producing a blending hot water heater which can maintain the temperature of blended water constant over a wide range of demand. To maintain the water leaving the blending chamber at a constant temperature the ratio between the superheated water arriving in the blending chamber and the cold water arriving in the blending chamber must be varied to compensate for two factors. First the pressure drop associated with the heat exchanger changes with the rate of flow through the heat exchanger. Second, the superheated water temperature tends to fall with increasing flow through the heat exchanger. Consequently, any controller which is to keep the blended water temperature constant must be able to compensate for the changes in these two factors with changing flow rates.
Further, many known control valve assemblies for blending hot water heaters have cumbersome external lines for conducting pressure to diaphragm controllers. These lines are not only difficult to install, they are prone to damage in service because they are generally made of a thin wall tubing and are in vulnerable locations.